A study of the metabolism of the branched-chain amino acids has revealed a pathway of catabolism of leucine in which the first step is catalyzed by leucine 2,3-aminomutase. The product, beta-leucine, is deaminated and cleaved to give acetate and isobutyrate. The enzyme found in bacteria, animals, plants, and man, is dependent upon coenzyme B12 and is stimulated by FAD, coenzyme A, DPN and pyridoxal phosphate. The enzyme has been partially purified and its preference for D-beta-leucine as substrate has ben established.